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Triboelectric Series of 2D Layered Materials

Minsu Seol, Seongsu Kim, Yeonchoo Cho, Kyung-Eun Byun, Haeryong Kim, Jihye Kim, Sung Kyun Kim, Sang-Woo Kim,* Hyeon-Jin Shin,* and Seongjun Park

Dr. M. Seol, Dr. Y. Cho, Dr. K.-E. Byun, Dr. H. Kim, Dr. H.-J. Shin, Dr. S. ParkSamsung Advanced Institute of TechnologySuwon 443-803, Republic of KoreaE-mail: [email protected]. S. Kim, J. Kim, Dr. S. K. Kim, Prof. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 440-746, Republic of KoreaE-mail: [email protected]. S.-W. KimSKKU Advanced Institute of Nanotechnology (SAINT)Sungkyunkwan University (SKKU)Suwon 440-746, Republic of Korea

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201801210.

DOI: 10.1002/adma.201801210

another material “B” ranked on a more positive side, “A” would acquire enhanced negative charge. The farthest the two materials are from each other on the series, the greater the charge transferred.

Recently, triboelectric nanogenerators (TENGs), which were developed by com-bining the concept of triboelectric charging and electrostatic induction, have received great attention as a new technology of energy harvesting. A TENG offers a unique and straightforward solution to convert mechanical energy into electricity.[5–7] Furthermore, using a similar principle, triboelectric sensors[6–8] and tribotronics[9,10] that utilize triboelectric output to drive and control electronic devices were also intro-duced. One of the important factors that determine the performance of the device is the material. However, materials known to exhibit triboelectric charging behavior are

only limited to some polymers and a few metals already located in the triboelectric series. Therefore, understanding the triboe-lectric charging behavior of new materials is important. Further investigating the triboelectric charging behaviors of different materials and widening the material library of the triboelectric series are required.

So far, 2D materials such as transition metal dichalcoge-nides (TMDs) and graphene (GR) have received great attention owing to their distinct properties. 2D materials are crystalline materials involving strong covalent bonds providing in-plane stability, and a relatively weak interlayer attraction facilitated by van der Waals forces, allowing their exfoliation from bulk form into individual, atomically thin layers. The isolated 2D materials exhibit particular properties different from their bulk counter-parts including electronic, optical, mechanical, and thermal properties, which have been investigated for decades.[11–15] Moreover, piezoelectric characteristics were also observed in monolayered TMDs that are non-centrosymmetric, because of strain-induced lattice distortion and subsequent charge polarization.[16–18] Meanwhile, triboelectric characteristics of 2D materials have been poorly studied. Dong et al. reported high-performance TENG device using fluorinated MXene,[19] and our group (Kim et al.) reported flexible, transparent TENG device using graphene.[7] Although 2D material-based TENG devices have been proposed in a few researches, the triboelec-tric charging behaviors of 2D materials have not been clearly understood.

In this study, we investigated the triboelectric charging behaviors of various 2D materials, including MoS2, MoSe2,

Recently, as applications based on triboelectricity have expanded, understanding the triboelectric charging behavior of various materials has become essential. This study investigates the triboelectric charging behaviors of various 2D layered materials, including MoS2, MoSe2, WS2, WSe2, graphene, and graphene oxide in a triboelectric series using the concept of a triboelectric nanogenerator, and confirms the position of 2D materials in the triboelectric series. It is also demonstrated that the results are obviously related to the effective work functions. The charging polarity indicates the similar behavior regardless of the synthetic method and film thickness ranging from a few hundred nanometers (for chemically exfoliated and restacked films) to a few nanometers (for chemical vapor deposited films). Further, the triboelectric charging characteristics could be successfully modified via chemical doping. This study provides new insights to utilize 2D materials in triboelectric devices, allowing thin and flexible device fabrication.

Triboelectric Nanogenerators

Triboelectric charging, an electrical charging phenomenon that occurs when different materials come into contact and separate spontaneously, is well known and has been studied for more than 2500 years.[1] Owing to the charge transfer during contact, charges of opposite signs accumulate on the surface of each material, thereby developing static electricity. Irrespective of the magnitude, all materials could be charged by contact. Some of the common materials consistently exhibit triboelectric charging patterns, and based on the charging characteristics, empirical “triboelectric series between materials” was constructed by arranging them according to the relative polarity of the contact charge acquired.[2–4] If material “A” contacts with

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WS2, WSe2, GR, and graphene oxide (GO), using the concept of TENG. The relative charging polarities of the 2D materials against materials in the conventional triboelectric series were observed by analyzing the output signals obtained from TENGs of various combinations. Consistent results of contact-charging polarities were observed, using which, a modified triboelectric series including 2D materials was developed. For further verifi-cation, the effective work functions of the 2D materials, which are one of the main factors deciding their triboelectric charging behaviors, were estimated via Kelvin-probe force microscopy (KPFM) and calculated via first principles simulations as well. Our study would provide new insights to utilize 2D materials in triboelectric devices, which could broaden their application area.

Each of the 2D nanosheets was prepared by chemical exfoliation of bulk flakes in a liquid phase, and the exfo-liated-restacked films were prepared by vacuum filtration (Figure S1, Supporting Information). It is well-known that layer–layer interactions can affect the electrical properties of 2D materials.[20] Here, the restacked film has minimized layer–layer interaction compared to the bulk counterpart, because it

consists of randomly arranged nanosheets with large spacings (Figure S2, Supporting Information). To investigate the position of various 2D materials in the conventional triboelectric series, the relative charging polarity of each 2D material against repre-sentative materials in the triboelectric series was measured by preparing simple pushing-type TENGs. Figure 1a shows a sche-matic diagram representing the device structure and operating mechanism of the fabricated TENG, using a MoS2–nylon pair. A flexible Cu foil was utilized both as electrode and substrate. The output voltage signal of the corresponding TENG is pre-sented in Figure 1b,c. Each section, designated as ①–④ in the magnified graph (Figure 1c), corresponds to each operating step shown in Figure 1a. Contact between two materials results in charge transfer from one to the other (Figure 1a③). During releasing, voltage induced between the two electrodes drives the electron flow from the negatively electrified side to the positively electrified one through the external load, so as to screen the remaining electrified charges on each contacted surface. Here, a negative voltage is observed during the releasing process (④ in Figure 1c), which drives the electrons flow from MoS2-side to the nylon-side (Figure 1a④); thus, we can conclude that MoS2

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Figure 1. Triboelectric charging behavior of MoS2. a) Schematic showing the device structure and working principle of the MoS2–nylon TENG. b) Output voltage signal of MoS2–nylon TENG and c) the magnified output voltage signal in one cycle. d) Triboelectric charging behavior of MoS2. The output voltage signals of TENGs with MoS2 and corresponding materials are shown underneath the triboelectric series. TEM images and the corresponding SAED patterns of e,f) (1T+2H)-MoS2 and g,h) 2H-MoS2. i) Raman spectra and j) XPS spectra showing Mo 3d peak region of (1T+2H)-MoS2 and 2H-MoS2.



is negatively electrified when contacted with nylon. During the pressing process, a positive voltage is observed (② in Figure 1c), indicating that the transferred electrons flow back in the reverse direction through the external load (Figure 1a②).

Similarly, we prepared several TENG combinations between a 200 nm thick MoS2 and six different well-known materials in the triboelectric series: (−) polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS), polycarbonate (PC), polyethylene terephthalate (PET), mica, and nylon (+). Figure 1d shows the triboelectric series of the materials used in this research. The tendency of a material to charge negatively upon contact increases from right to left. The output voltage signals of the TENGs with MoS2 and corresponding materials are exhibited underneath the triboelectric series. All the output results were measured during cyclic pressing and releasing, and the validity of the output signals was also confirmed by switching the polarity test by connecting in the reverse. The TENG with MoS2–PTFE pair exhibits a negative output during pressing and a positive output during the release process. This has an opposite polarity to that of the MoS2–nylon pair (Figure 1c), indicating that MoS2 is positively electrified upon contacting with PTFE. On the contrary, TENGs with MoS2–PDMS, MoS2–PC, MoS2–PET, and MoS2–mica pairs exhibit the same polarity of the output signal as that of the MoS2–nylon pair, indicating that MoS2 is negatively electrified upon contact. Thus, it is clear that MoS2 lies between PTFE and PDMS in the triboelectric series, as shown in Figure 1d.

In general, the TMDs have three main polymorphs in nature or synthetic products, which are 1T, 2H, and 3R. To investigate the effect of polytype on the triboelectric charging behaviors of the TMDs, we fabricated two types of MoS2 nanosheets. It is difficult to prepare pure 1T-MoS2 in large scale due to its metastable characteristics. Instead, MoS2 nanosheets with the (1T+2H)-mixed phase were prepared by lithium-interca-lation and forced hydration of bulk MoS2.[21,22] 2H-MoS2 was obtained by annealing as-prepared (1T+2H)-MoS2.[21,23] 3R polytype is found only in bulk form and easily relaxes to the 2H phase, thus 2H polytype was only prepared for comparison. Figure 1e–h shows transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns of exfoliated MoS2 nanosheets before and after annealing. Change in the SAED pattern from √3a × a-type superstructure (Figure 1f, owing to the periodic lattice distortion from the a × a unit cell and the presence of 1T phase)[21,22] to a typical a × a type sixfold symmetry (Figure 1h) indicates the phase tran-sition. Moreover, as shown in the Raman spectra (Figure 1i), additional peaks designated as J1–J3 that reveal the presence of 1T phase disappeared after annealing, indicating the phase transition to 2H-MoS2.[24] Deconvolution of the Mo 3d peak in the X-ray photoelectron spectra (XPS, Figure 1j) also reveals the presence of each polytype in the as-prepared film and the tran-sition to 2H-MoS2 after annealing.[24] Figure 1d and Figure S3 (Supporting Information) show the triboelectric charging behaviors of (1T+2H)-MoS2 and 2H-MoS2, respectively. Empir-ical results of TENGs exhibit the same output polarity for both cases, indicating that the position of MoS2 does not change in the triboelectric series depending on its polytype.

Although most of the TMDs have similar crystal structure, they have different band structures as well as bandgaps,

depending on the composition. To identify their triboelec-tric charging behaviors, the nanosheets of MoSe2, WS2, and WSe2 were also prepared and investigated in the same way of MoS2. Raman spectra (Figure 2a) were obtained for all sam-ples, and the expected A1g mode and E1

2g mode are respectively observed at 239 and 287 cm−1 for MoSe2 and 417 and 353 cm−1 for WS2.[25] For WSe2, only a single broad peak is observed at ≈251 cm−1, as a result of the degeneration of the A1g and E1

2g modes in few-layered WSe2.[25,26] The structure of the TMD was also confirmed by XPS (Figures S4 and S5, Supporting Infor-mation). Figure 2b–d and Figure S6 (Supporting Information) show the triboelectric charging behaviors of MoSe2, WS2, and WSe2, respectively. According to the output signal, the polarity of the signal for MoSe2 changes between PTFE and PDMS, same as the case with MoS2. On the contrary, WS2 and WSe2 exhibit polarity changes between PDMS and PC, indicating that they are triboelectrically positive compared to MoS2 and MoSe2. Moreover, other representative 2D materials, GR and GO, were also prepared for comparison. Figure 2e shows the Raman spectra of GR and GO, and the peaks are characterized with D, G, and 2D peaks located at 1345, 1575, and 2700 cm−1, respec-tively. Compared to GO, GR exhibits a sharp 2D peak, indi-cating its well-exfoliated high-quality structure. The triboelectric charging behaviors of GO and GR are shown in Figure 2f,g and Figure S7 (Supporting Information). They also show polarity changes between PDMS and PC, same as in the case of WS2 and WSe2. Based on these empirical results, the relative posi-tions of the 2D materials in the conventional triboelectric series are defined. All the 2D materials used in this study are located near the negative side of the triboelectric series, indicating that they could be easily electrified with a negative charge. MoS2 and MoSe2 exhibit the most negative triboelectric charging characteristics among the 2D materials investigated in this work, with their position located between PTFE and PDMS. Other 2D materials including WS2, WSe2, GR, and GO exhibit the same signal polarity patterns, and they are located between PDMS and PC.

To confirm the exact order of the 2D materials in the series, the output values of TENGs consisting of 2D materials at one side and nylon at the other side were compared. Nylon is known as one of the most positive materials in the triboelectric series and all 2D materials used here exhibited more negative contact-charging characteristics than nylon. Some parameters of the 2D films including the thickness, surface roughness, dielectric constant, surface density of state (DOS), as well as surface contamination could affect the output values of TENGs. Therefore, assuming these parameters are similar for all 2D films, the combination of nylon with more negative triboelectric material could generate more triboelectric charges and thus the TENG could exhibit a higher output. Here, all 2D films were prepared with the similar thickness of 200 nm, by controlling the concentration of the dispersion before filtering. Figure S8 (Supporting Information) shows the atomic force microscope (AFM) images of the 2D films, indicating that all the 2D films transferred on Cu foils exhibited a similar surface roughness, except for WSe2. WSe2 film had a rough surface, possibly due to remaining unexfoliated bulk flakes.

Figure 3a exhibits the output voltage of the 2D material–nylon TENGs with a force of 0.3 kgf at a frequency of 1 Hz. The

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Figure 3. Output results of 2D-nylon TENGs and relative triboelectric characteristics of the 2D materials. a) Output voltage signals of 2D material–nylon TENGs. b) Scheme for the mechanism of contact charge transfer. c) Quasi-Fermi level positions of the corresponding 2D materials estimated via KPFM.

Figure 2. Triboelectric charging behaviors of other 2D materials. Raman spectra of a) MoSe2, WS2, and e) WSe2, and GR and GO. Triboelectric charging behaviors of b) MoSe2, c) WS2, d) WSe2, f) GR, and g) GO. Output voltage signals of the corresponding TENGs are shown underneath the triboelectric series.



output currents of the corresponding TENGs are also shown in Figure S9 (Supporting Information). MoS2 exhibits the highest output voltage and current values reaching up to 7.48 V and 0.82 µA, respectively, indicating that MoS2 is triboelectrically the most negative among the 2D materials examined in this study. MoS2 also exhibits the highest power density of 36.3 mW m−2 at the external load resistance of 10 MΩ for impedance matching compared to other TMDs (Figure S10, Supporting Information). It is a comparable performance of TENGs by conventionally used materials in triboelectric series based on similar device structure.[27] 2D materials could be listed in the order of MoSe2, GR, GO, WSe2, and WS2 according to the maximum output voltage and current. As the ordering of the output power matches well with the ordering of the maximum output voltage and current, based on these TENG output results, we can pre-dict the triboelectric ordering between the 2D materials to be as follows: (−) MoS2, MoSe2, GR, GO, and WS2 (+). It is hard to estimate the relative triboelectric charging characteristics of WSe2 from the output results of TENGs, because their surface roughness was different compared to those of the other 2D films. In case of 2D material–PTFE TENGs, The output voltage shows the opposite behavior compared to those of 2D material–nylon data (Figure S11, Supporting Information).

To clarify the triboelectric order between the 2D materials derived from TENG output results, the effective work function of each material was compared. For a metal–metal contact, many studies have shown that the difference in the work function leads to contact potential difference (CPD), which drives the electron transfer to align Fermi level.[1] For semiconductors and insulators, a similar concept can be used to explain the electron transfer by assuming that there are localized electronic states near the surface and that electrons are exchanged to equilibrate the effective work function.[1] Figure 3b illustrates the contact charging mechanism between the 2D material and the polymer when they have different effective work functions, φ1 and φ2, respectively. If a 2D material has a higher work function than that of the polymer, (φ1 > φ2), electron transfer will occur from the filled electronic states of the polymer to the empty electronic states of the 2D material when they are brought into contact, so as to equalize the Fermi levels of the two materials. Therefore, the direction of the charge transfer is determined by the com-parison of the effective work function values, and the amount of charge transfer is dominantly affected by the amount of surface DOS as well as the difference of effective work function.[1,28–31] In this work, we focused on their effective work function for observing the triboelectric series. For more clear understaning of 2D triboelectrification phenomenon, additional theoretical study of surface DOS of 2D materials is needed for future work. Here, the effective work functions of the 2D materials (φ2D) were estimated via KPFM. The CPD between the probe and the 2D material could be obtained from the KPFM mode of AFM. Then, φ2D is defined as

eV2D probe CPDφ φ= − (1)

where φprobe is the work function of the probe, VCPD is the measured CPD, and e is the electronic charge. In Figure 3c, the positions of the Fermi levels of the 2D materials are illustrated in the order of decreasing magnitude of the effective

work function. MoS2 exhibits the highest effective work func-tion, 4.85 eV. MoSe2 exhibits a slightly lower value of 4.70 eV and is thus expected to be triboelectrically more positive than MoS2. Other materials including GR, GO, WS2, and WSe2 exhibit effective work functions of 4.65, 4.56, 4.54, and 4.45 eV, respectively. WSe2 has the lowest work function, indicating that it is the most positive material among the 2D materials investigated in this work. Except for WSe2 which has different surface roughness, output voltage of the 2D material–nylon TENG is gradually increased depending on the work function of the corresponding 2D materials (Figure S12, Supporting Information). Comparable output was observed in 2D mate-rial–nylon TENGs, indicating that 2D materials could easily be charged by contact, thus they must contain proper surface DOS for charge transfer. The nonlinearity might be originated from the difference in the surface DOS between 2D materials.[1,28–31]

We performed first-principles simulations to understand the differences between the effective work functions and the resulting triboelectric series of 2D TMDs. In order to estimate the effective work functions, we computed the electron affini-ties (EA), ionization potentials (IP), and single-vacancy (SV) levels of TMD monolayers in the 2H phase (Figure S13 of the Supporting Information and Table 1). The SV levels are likely to govern the effective work function, since the SV is the most prevalent point defect of TMD materials in the 2H phase. We discovered that the computed SV levels are ≈0.3–0.6 eV lower than the corresponding effective work functions estimated by KPFM, with a similar ordering of the 2D materials. MoS2 and WSe2 exhibited the highest and lowest SV levels, with values of 4.56 and 3.79 eV, respectively, revealing yet again that they present the most negative and most positive triboelectric charging characteristics among the 2D materials. MoSe2 and WS2 have similar SV levels at 4.15 and 4.17 eV. Although their order is not matched with the values measured by KPFM, the difference between their SV levels is within the range of calcu-lation errors, and it might be due to the other types of defects contributing to the effective work function.

To verify the validity of the results in 2D films with different thickness and materials synthetic method, we prepared few-layered MoS2 by chemical vapor deposition (CVD) and inves-tigated its triboelectric charging behavior in the same fashion. Sometimes characteristics of the underlying substrate might be reflected in case of a 2D monolayer.[32] In our previous research, we revealed that the triboelectric charging behavior might be different in a 2D monolayer (e.g., tunneling tribo-electrification phenomenon).[33] Here, we investigated the triboelectric charging behavior of 3-layer MoS2 (3L-MoS2) for excluding the substrate effect. Fully covered, polycrystalline MoS2 with a domain size of a few tens of nanometers were

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Table 1. Computed properties of the TMD monolayers. All TMDs are in the 2H phase (HSE06).

TMD monolayer MoS2 MoSe2 WS2 WSe2

EA 4.11 3.73 3.74 3.36

IP 6.35 5.72 6.10 5.48

Gap 2.23 1.99 2.37 2.12

SV level 4.56 4.15 4.17 3.79



grown directly on SiO2, as shown in the top-view TEM image in Figure 4a. The prepared MoS2 film has an average layer number of 3 (3L-MoS2, ≈2 nm), as shown in the cross-sectional TEM image (Figure 4b). The expected E1

2g and A1g modes of MoS2 are observed at 385 and 407 cm−1 in the Raman spectrum (Figure 4c); these are similar to the Raman peaks of exfoliated and annealed MoS2 (Figure 1i). Triboelectric charging polarities of thin, CVD-grown MoS2 films (3 layers, ≈2 nm) are same as those of thick, exfoliated MoS2 films (≈300 layers, ≈200 nm). Even at a different thickness of CVD-grown MoS2 (7 layers, 5 nm) the same polarity change was observed (Figure S14, Supporting Information). Moreover, the surface character-istics of the Cu substrate were seldom reflected in this result (Figure S15, Supporting Information), even in the case of very thin MoS2 layers. According to the simulation, the computed values of the gap and IP decreased and EA increased upon increasing the layer number (Table S1, Supporting Informa-tion). However, the position of the SV level is almost constant regardless of the number of layer, indicating that the posi-tion of each material in the triboelectric series should remain unchanged. As a result, we can conclude that the triboelectric polarity characteristics are independent of the thickness of the MoS2 and the synthetic method used to prepare it, even at few nanometer scales.

Chemical doping is a simple process to modify the electrical properties of 2D materials. Gold chloride (AuCl3) and benzyl viologen (BV) are representative p-type and n-type chemical-dopants used in 2D material research.[34–36] These materials could modulate the work function of 2D materials. Figure 4e,f

and Figure S16 (Supporting Information) exhibit the tribo-electric charging characteristics of p-type MoS2 obtained via AuCl3 doping (AuCl3-doped MoS2) and n-type MoS2 obtained via BV doping (BV-doped MoS2). Chemical doping with AuCl3 increases the effective work function (Figure S17, Sup-porting Information), but empirical results show that AuCl3-doped MoS2 is still located in a more positive position than PTFE. Doping with BV decreases the effective work function (Figure S17, Supporting Information) and thus successfully changes the position of MoS2 in the triboelectric series, to lie in between PDMS and PC. The triboelectric charging characteris-tics could be successfully modified via chemical doping, which changes the effective work function.

In summary, according to the empirical results of 2D material-based TENGs and the effective work function values, we obtained the modified triboelectric series including 2D materials (Figure 5). MoS2 exhibited the most negative triboelectric charging characteristics among the 2D materials studied and all the 2D materials were located near the nega-tive side of the triboelectric series. Interestingly, it was found that the charging polarities of the MoS2 samples are inde-pendent of the synthetic method and their thickness ranging from a few hundred nanometers (chemically exfoliated and restacked films) to a few nanometers (CVD-grown films). Instead the triboelectric charging characteristics of MoS2 could be successfully modified via chemical doping. This study is highly expected to pave the way for the use of 2D materials with only atomic-scale thickness in triboelectric devices, which could broaden their application area.

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Figure 4. Triboelectric charging behavior of CVD-grown, few-layered MoS2. TEM images of 3L-MoS2: a) Top-view and b) cross-section. c) Raman spectrum of 3L-MoS2. Triboelectric charging behaviors of d) pristine 3L-MoS2, e) AuCl3-doped MoS2, and f) BV-doped MoS2. Output voltage signals of the corresponding TENGs are shown underneath the triboelectric series.



Experimental Section2D-Based Film Preparation: Exfoliated TMD nanosheet dispersions

were prepared by Li-intercalation of the corresponding bulk materials, followed by forced hydration. TMD bulk flake (350 mg) was immersed in 3.5 mL of n-butyllithium in hexane (1.6 m). For MoS2 (Sigma-Aldrich, particle size ≈ 6 µm), the mixture was stirred for 48 h at room temperature under nitrogen atmosphere. For other TMDs including MoSe2 (Materion, ≈10 µm), WS2 (Sigma-Aldrich, ≈2 µm), and WSe2 (Materion, ≈5 µm), the mixture was stirred for 24 h at 80 °C under nitrogen. Then, the resultant Li-intercalated mixture was filtered and rinsed with hexane. 300 mL H2O was added to the product remaining on the filter paper, and the resulting suspension was sonicated in a sonic bath (Bandelin RK 106) for 1 h to achieve exfoliation by forced hydration. Then, the suspension was filtered over a PTFE filter and then rinsed with H2O. The exfoliated nanosheets remaining on the filter were dispersed in 100 mL dimethylformamide (DMF) via sonication. The dispersion was centrifuged at 8000 rpm for 20 min and the supernatant was collected to remove unexfoliated bulk materials. GO was prepared by a modified Hummers method.[37] GR was prepared by ClF3-intercalation and exfoliation.[38] An exfoliated restacked film was obtained by vacuum filtering a 2D nanosheet suspension over an anodisc membrane, followed by drying in a vacuum oven at 60 °C. The anodisc membrane was etched using 0.1 m NaOH, and the remaining free standing film was rinsed with H2O and transferred to a Cu foil.

For comparison, a few-layered MoS2 film was prepared by CVD, on a SiO2/Si substrate. The SiO2 layer was etched slightly using hydrogen fluoride (HF), and then the free-standing MoS2 film was separated and rinsed with H2O and transferred onto the Cu foil substrate. Chemical doping of MoS2 was carried out by spin-coating. The AuCl3 dopant solution was prepared by dissolving AuCl3 powder in nitromethane (20 × 10−3 m). The BV dopant was prepared by dissolving 1,1′-dibenzyl-4,4′-bipyridinium dichloride powder in deionized water, followed by the addition of toluene and sodium borohydride (NaBH4) to reduce and transfer BV to the toluene phase. The toluene phase was extracted from the solution after 1 d. Each dopant solution was spin-coated onto MoS2 at 4000 rpm for 1 min, followed by annealing at 100 °C for 10 min.

Characterization: AFM and KPFM measurements were performed using an E-sweep AFM system (Seiko Instruments Inc.) with Rh-coated probes (SI-DF3-R, Seiko Instruments Inc.). Surface charges were measured in the KPFM mode at an oscillation amplitude of 5 V and resonance frequencies in the range of 24–26 kHz. All the measurements were performed in dry N2 at room temperature. Raman spectroscopy was conducted with RM-1000 Invia (Renishaw) system, via a 514 nm Ar− ion laser. TEM and XPS were carried out on Tecnai Osiris (FEI) and Quantum 2000 (Physical Electronics). Triboelectric characterization was performed by preparing pushing-type TENG devices. TENG devices were prepared with 2D-based films (MoS2, MoSe2, WS2, WSe2, GO, and GR) and counter materials (PTFE, PDMS, PC, PET, mica, and nylon). Cyclic contact and separation between the two plates was accomplished by a force simulator (Z-TEC ZPS 100). The gap between the two plates was 1 mm, and the contact area was 0.785 cm2. The vertically compressive force applied was 0.3 kgf. The output currents and voltages generated by the device were measured using an oscilloscope (Tektronix DPO 3052) and a picoammeter (Keithley 6485). All the TENG output was measured in the box system purged with dry N2, in order to control the humidity and surface contamination during

the measurement. Moreover, air conditioning system was used to control the temperature and humidity of the laboratory.

First-Principles Simulations: The Vienna Ab initio Simulation Package was employed to compute the electronic properties of the 2D materials.[39] The plane wave cutoff was set to 400 eV. The geometries were relaxed using the Perdew–Burke–Ernzerhof functional with D3 correction.[40,41] All the electronic properties were computed using the HSE06 functional. To calculate the electron affinity and ionization potential, the number of sampled k-points was increased until the total energy was converged within 1 meV per atom. For calculating the properties of a single vacancy, only the gamma point was sampled. However, the convergence within 0.01 eV was verified by comparing it to the result obtained by employing k-points on a 5 × 5 grid.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsM.S. and S.K. contributed equally to this work. This work was financially supported by the Industrial Strategic Technology Development Program (Grant No. 10052668, Development of wearable self-powered energy source and low-power wireless communication system for a pacemaker), the Technology Innovation Program (Grant No. 10065730, Flexible power module and system development for wearable devices), and the “Human Resources Program in Energy Technology (Grant No. 20174030201800)” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry & Energy, (MOTIE, Korea).

Conflict of InterestThe authors declare no conflict of interest.

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Figure 5. Modified triboelectric series including 2D materials. The molecular structure is shown on the right of the corresponding material.



Adv. Mater. 2018, 1801210

Keywords2D materials, chemical doping, effective work function, triboelectric nanogenerators, triboelectric series

Received: February 21, 2018Revised: July 15, 2018

Published online:

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